The present invention relates to power supplies for supplying regulated DC voltages and more particularly to a switch mode DC to DC inverter with: an improved input filter which provides protection against high voltage excursions or spikes at the input to the inverter; a high voltage multiplier coupled to the inverter by a unique coupling circuit which stabilizes and provides a more closely regulated high voltage output; and an improved high voltage transformer.
Nowadays, practically all electrical instruments and appliances incorporate logic, control and computer circuits and/or display devices such as CRT's and the like. These circuits and devices must be provided with power in the form of one or more well regulated and stable DC voltages.
The DC voltages are generated with power supplies, sometimes referred to as power processors, which derive electrical power from a comparatively unregulated AC or DC input voltage source having a nominal voltage level. The power processors convert the source voltage to the one or more required DC voltages and carefully regulate the generated voltages to assure that they meet a predetermined set of specifications, regardless of fluctuations in the input voltage, loading, etc.
By way of example, the main power source in many airplanes is a DC voltage of 28 volts which is distributed throughout the airplane. Where needed, power processors are provided to receive this relatively poorly regulated DC power, to convert it to, for example, a well regulated 5 volt DC output for driving logic/computer circuits, +/-15 volts DC outputs for various analog circuits or any other DC voltage which a particular application may require.
Certain devices require very high DC voltages. A CRT display is a good example. It requires a DC voltage of between 5 to 25 kilovolts. Such an output can also be provided from the 28 volt DC source.
In recent automobile models power processors convert the widely fluctuating input voltage which is provided from the automobile's 12 volt battery to other stable and well regulated DC voltages for powering sensitive electrical circuits such as computer, logic or control circuits.
Power processors may include in them a regulator, in the form of a DC to DC inverter and a regulator circuit therefor, the overall regulator being operable either linearly or in switch mode. Switch mode regulators are relatively more complex. They operate on the principle of "chopping" an input DC source voltage into an "anti-phased" square wave signal having a voltage level that enables the square wave to be rectified to obtain a desired DC voltage level. Switch made inverters/regulators generate electromagnetic interference and voltage spikes which require more complicated and costly filters, as compared to purely linear regulators.
Nonetheless, switch mode inverters are popular because of their ability to provide significantly higher power conversion efficiencies and because of their smaller physical dimensions.
Switch mode power processors are often exposed to applications in which the processors must handle source input DC voltages which are subject to substantial transients. For example, in aircraft, automobiles, factories and like, equipment such as lighting ballasts, motor controls and similar inductive loads are powered from the same primary power source from which the regulated DC voltage is produced. This generates abnormal input transient voltages and energies which propagate into the power processors and which could expose the sensitive electronic components in them to voltage levels which are substantially beyond the maximum voltage rating of such components. Such input transients may also be induced by lightning, lamp ignitors, inductive loads or the like.
Protection against input transients is conventionally provided by expensive clamping components such as metal oxide varistors, transient absorbing zeners and a variety of filtering elements. The known protection methods are heavily dependent upon the clamp ratio which is associated with the selected clamping/transient-absorbing device. The clamp ratio is the ratio of the voltage at which the protective device begins to conduct to the voltage which appears across the protective device when the maximum current is flowing through the device. Ratios of 1.42 to 2 are not uncommon. This means that an excessive voltage is still present at the protection device, even while the device operates in its protective mode. The energy of this excessive voltage is able to leak into and damage the electronic components within the power processor. The only choice is to use overspecified and more expensive switching elements (transistors) in the power processor, which is undesirable.
Further, in given applications, inverters are designed to generate a very high DC voltage output for powering cathode-ray tube (CRT) displays, photomultipliers and the like. The output voltage may be in the range of 5 to 25 kilovolts or even higher. If the inverter generates only the high voltage output it is common to include a feedback network between the high voltage output and the controller for the inverter. A feedback network increases the cost and the complexity of the inverter.
Other DC inverters are known which generate both low voltage outputs and a high voltage output. If only the low voltage main outputs are stabilized and the design relies on the ability of the high voltage output to "track" the regulation that is applied to the low voltage output, the high voltage output may be only loosely regulated. The reason for this may be explained as follows.
Generally, a DC to DC inverter operates at ultrasonic frequencies and a high voltage transformer is deployed for stepping up the AC square wave output of the inverter to the required high voltage level which is then rectified and filtered to obtain the high DC output. High voltage transformers for stepping up a relatively low voltage to a multi-kilovolt output have very high secondary to primary winding ratios, for example, 50 to 100:1. These high-ratios magnify the effects of the inherent shunt capacity and leakage inductance present in all transformers which adversely affect the inverter to which the transformer is coupled. Designers have, therefore, turned to transformers with lower turn ratios.
To obtain the necessary multi-kilovolt output, the technique of voltage multiplication using a network of multiple diodes and resistors which are connected in accordance with the well known Crockoft-Walton method for voltage multiplication have been used by those skilled in the art. Voltage multipliers provide however relatively poor load regulation. The poor regulation is attributed in part to the presence of a parasitic shunt capacitance in the transformer. This shunt capacitance stores energy and delivers it to the voltage multiplier cyclically. During the transition intervals of the square wave waveform of the inverter, voltage overshoots are created due to the formation of a tuned circuit with the parasitic and discreet inductances and capacitances which are elsewhere present in the power processor.
The net result is voltage overshoot and ringing which causes the voltage multiplier to charge to the peak of these overshoot/ringing voltages, particularly when the output load is relatively light. On the other hand, under heavy loads, the charging peaks are damped and the output voltage falls, producing a comparatively poorly regulated output.
The voltage overshoot and ringing effect results in part from the shunt/stray capacitance at a secondary of the high voltage transformer which is reflected into the primary winding of the driving inverter. This shunt/stray capacitance is seen at the primary winding as a larger capacitance due to the magnification of this shunt capacitance by the square of the transformer turns ratio. It therefore presents to the inverter a highly capacitive load. This is undesirable because it increases switch stresses in the inverter and unnecessarily consumes current which could otherwise be drawn by the low voltage loads of the inverter.
Although the foregoing effects can be reduced with a resistor connected in series with the primary of the high voltage transformer, the solution is impractical as it causes power loss and greatly reduces efficiency.
The above-mentioned shunt capacitance of a transformer poses a problem for any design which incorporates transformers, but especially for high voltage/high frequency transformers in inverter applications. The shunt capacitance is produced by the insulation on wires and from insulating materials in the transformer which are charged by voltage fields within the transformer. Since, in switch mode inverters, the transformer primary winding is driven by a high frequency square waveform, with each transition of the square wave, the transformer shunt capacitance is charged and discharged, alternatively storing and delivering energy. This energy transfer causes the overshoots and the ringing which adversely affect voltage regulation and power conversion efficiency.
The secondary winding of a high voltage transformer has many turns of relatively fine wire and this produces a significant self capacitance. Typically this self capacitance could be 10 to 100 picofarads. A capacitance of this magnitude stores considerable energy in view of the high voltage charging it, in accordance with E=1/2CV.sup.2.
High voltage transformers are particularly susceptible to self capacitance effects because the energy stored in them is proportional to the square of the voltage multiplied by the capacitance. Therefore, as the transformer turns ratio is increased, the stored energy increases with the square of the turns ratio.
The art identifies two types of capacitances with high voltage transformers. One is an interwinding capacitance, consisting of the capacitance between the individual turns of the transformer. The other is the winding-to-ground capacitance from each turn of the transformer, with the core being considered as ground. Since one side of the high voltage transformer is generally returned to ground, both of these shunt capacitances appear to load the transformer's secondary winding.
Known techniques for solving the problem of self capacitance in a high voltage transformer include increasing the radial dimension of the transformer, separating the windings into many sections and employing universal windings. The known techniques increase leakage inductance and the size of the transformer.